The present invention relates to a driving system for driving a self-luminous display.
There has been known a method for displaying an image on a plasma display panel as a self-luminous display by controlling a tone of the image. In the method, each field of a video signal is divided into N pieces of sub-fields, and each sub-field emits the light for a time length corresponding to a weight applied to each bit of N-bit pixel data.
If a pixel data for each pixel has 8 bits, each field is divided into eight sub-fields, SF8, SF7, SF6, . . . , SF1 in descending order of weight from the first to the eighth. The time length of each sub-field is determined in accordance with its weight. The sub-fields SF8 to SF1 emit the light by 128 pulses, 64 pulses, 32 pulses, 16 pulses, 8 pulses, 4 pulses, 2 pulses and 1 pulse, respectively, in order. Thus, the tone of 256 steps can be obtained by combining the eight sub-fields.
For example, if a pixel data applied to a pixel is "10000000" (light emitting pattern), only the sub-field SF8 emits light in a field. During the sub-field SF8, the light is emitted by 128 pulses, thereby obtaining a display having the tone of 128 levels.
Furthermore, if the pixel data is "10100000", the sub-fields SF8 and SF6 emit light in the field. During the sub-fields SF8 and SF6, the light is emitted by 128 and 32 pulses, thereby obtaining a display having the tone of 160 (128+32) levels.
In such a method, the sub-fields SF8 to SF1 start emitting the light in a fixed order, from the sub-field having the longest time length, or from the sub-field having the shortest time length.
However, when the tone level is decreased from 128 to 127, namely, the pixel data changes from "10000000" to "01111111", false contours of stripes are recognized as if the tone of the image is lost, in spite of gradual change of luminance and tone. Therefore, a problem that the quality of display is extremely deteriorated arises. Namely, when the pixel data changes from "10000000" to "01111111", the emission of light does not occur for a period between the sub-field SF7 of the first field and the sub-field SF8 of the second field. Therefore, the luminance momentarily reduces.
Japanese Patent Application Laid-Open No. 7-271325 discloses a method for display an image by controlling a tone. In order to restrain reduction of the quality of display, a sub-field having a large weight is further divided into a plurality of parts, and these parts are separately disposed in a field.
FIG. 17 shows a light emitting format in the field disclosed in the method. The field is divided into six sub-fields, SF6 to SF1 having the ratio of weights 32:16:8:4:2:1 in order. The heaviest sub-field SF6 is divided into short sub-fields SF6.sub.1 and SF6.sub.2 each of which has the ratio of weight 16. Furthermore, the second heavier sub-field SF5 is divided into short sub-fields SF5.sub.1 and SF5.sub.2 each of which has the ratio of weight 8. Thus, in the field, three sub-fields SF4, SF5.sub.1 and SF5.sub.2 of the ratio of weight 8 and two sub-fields SF6.sub.1 and SF6.sub.2 of the ratio of weight 16 are provided.
Here, the pixel data for each pixel is converted into a light emitting pattern shown in FIG. 17. The pixel data having the level of 32nd of luminance and tone is converted into one of the patterns as follows.
"01010001": light emitting pattern A PA1 "10010010": light emitting pattern B
FIG. 18a shows a field having the light emitting pattern A where the light is emitted only during the short sub-fields SF6.sub.1, SF4 and SF5.sub.2. The light is not emitted during the other sub-fields (hatched areas). On the other hand, FIG. 18b shows a field having the light emitting pattern B where the light is emitted only during the sub-fields SF5.sub.1, SF4 and SF6.sub.2.
During both of the fields, although the light is emitted to obtain the tone level of 32nd, the timings for emitting the light differ.
In the method, the light is emitted by the light emitting patterns A and B which are alternately performed every pixel on the display.
Therefore, when the pixel data changes from "10000000" to "01111111", namely, at a border where a figure of the high-figure bit of the pixel data is down, sequential non-light emitting period as aforementioned does not occur. Thus, the false contour is reduced when the image is moved on the display.
However in such a method, when watching a moving image on the display, a further problem of trouble in that dark and bright stripes may appear on the display arise.
FIG. 19 shows an image having the tone level of 32nd for explaining the trouble in which the image moves from an upper portion to a lower portion of a picture. G11 to G81 are pixels shown in FIG. 18 only from the first row to the eighth row on the first column in the picture. The field of each pixel has the format shown in FIG. 17. Each of the pixels G11, G31, G51 and G71 on odd rows has the field emitted by the light emitting pattern A shown in FIG. 18a. Each of the pixels G21, G41, G61 and G81 on even rows has the field emitted by the light emitting pattern B shown in FIG. 18b.
As shown in FIG. 19, if a viewer moves his eyes as shown by an arrow S1 together with the image having the tone level of 32nd, a dark stripe having a lower tone level than that of 32nd is recognized. Namely, the non-light emitting period is recognized longer than the light emitting period because of the timing of the arrow S1.
To the contrary, if the viewer moves his eyes as shown by an arrow S2, a bright stripe is recognized. Namely, the light emitting period is recognized longer than the non-light emitting period because of the timing of the arrow S2.